Trivalent chromium plating solution and method for chromium-plating using same

ABSTRACT

A trivalent chromium plating solution having a high plating deposition rate and being practical is provided with a trivalent chromium plating solution containing a trivalent chromium compound, a complexing agent, potassium sulfate and ammonium sulfate as a conductive salt, a pH buffer, and a sulfur-containing organic compound, containing a carboxylic acid having two or more hydroxy groups and two or more carboxy groups or a salt thereof as the complexing agent, and containing a combination of saccharin or a salt thereof and a sulfur-containing organic compound having an allyl group as the sulfur-containing organic compound.

TECHNICAL FIELD

The present invention relates to a trivalent chromium plating solutionand a method for chromium-plating using the same.

BACKGROUND ART

Chromium plating is used as a coating film for decoration owing to thesilvery white color thereof. Hexavalent chromium has been used for thechromium plating, but the use of hexavalent chromium is restricted inrecent years due to the environmental implications thereof, and thetechnology is shifted to the use of trivalent chromium.

Many reports have been made for the technique using trivalent chromium,and for example, a chromium electrolytic plating solution containing awater soluble trivalent chromium salt, a complexing agent for atrivalent chromium ion, such as malic acid, a pH buffering compound, asulfur-containing organic compound, such as thiourea, and a watersoluble compound, such as saccharin, and having pH of from 2.8 to 4.2has been known (PTL 1).

However, this trivalent chromium plating solution has a low platingdeposition rate and thus is not practical.

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 5,696,134

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a trivalent chromiumplating solution that has a high plating deposition rate and ispractical.

Solution to Problem

As a result of the earnest investigations by the present inventors, ithas been found that a trivalent chromium plating solution can have ahigh plating deposition rate and can be practical by using a carboxylicacid having two or more hydroxy groups and two or more carboxy groups ora salt thereof as a complexing agent for a trivalent chromium ion, andin addition, using a combination of saccharin or a salt thereof and acompound having an allyl group as a sulfur-containing organic compound,and thus the present invention has been completed.

Accordingly, the present invention relates to a trivalent chromiumplating solution containing a trivalent chromium compound, a complexingagent, potassium sulfate and ammonium sulfate as a conductive salt, a pHbuffer, and a sulfur-containing organic compound,

containing a carboxylic acid having two or more hydroxy groups and twoor more carboxy groups or a salt thereof as the complexing agent, and

containing a combination of saccharin or a salt thereof and asulfur-containing organic compound having an allyl group as thesulfur-containing organic compound.

The present invention also relates to a method for chromium-plating onan article to be plated, including electroplating an article to beplated with the trivalent chromium plating solution.

The present invention further relates to a chromium-plated productincluding an article to be plated, electroplated with the trivalentchromium plating solution.

Advantageous Effects of Invention

The trivalent chromium plating solution of the present inventionprovides an appearance that is equivalent to plating with hexavalentchromium despite the plating with trivalent chromium, and furthermorehas a high plating deposition rate and is practical.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration showing the position where the throwingdistance is measured in the Hull cell test of Example 1.

FIG. 2 is a graph showing the relationship between the ratio ofpotassium sulfate and ammonium sulfate contained in the trivalentchromium plating solution and the throwing power in the Hull cell testof Example 1.

FIG. 3 is images showing the results of the corrosion resistance test(CASS test).

DESCRIPTION OF EMBODIMENTS

The trivalent chromium plating solution of the present invention (whichmay be hereinafter referred to as a “plating solution of the presentinvention”) is a trivalent chromium plating solution that contains atrivalent chromium compound, a complexing agent, potassium sulfate andammonium sulfate as a conductive salt, a pH buffer, and asulfur-containing organic compound,

contains a carboxylic acid having two or more hydroxy groups and two ormore carboxy groups or a salt thereof as the complexing agent, and

contains a combination of saccharin or a salt thereof and asulfur-containing organic compound having an allyl group as thesulfur-containing organic compound.

The trivalent chromium compound used in the plating solution of thepresent invention is not particularly limited, examples of which includebasic chromium sulfate, chromium sulfate, chromium chloride, chromiumsulfamate, and chromium acetate, and basic chromium sulfate and chromiumsulfate are preferred. The trivalent chromium compound may be used aloneor as a combination of two or more kinds thereof. The content of thetrivalent chromium compound in the plating solution of the presentinvention is not particularly limited, and for example, is from 1 to 25g/L, and preferably from 5 to 15 g/L, in terms of metallic chromium.

The complexing agent used in the plating solution of the presentinvention is a carboxylic acid having two or more hydroxy groups and twoor more carboxy groups or a salt thereof. Examples of the complexingagent include a carboxylic acid, such as tartaric acid, and a salt ofthe carboxylic acid, such as diammonium tartarate, Rochelle salt, andsodium tartarate. The complexing agent may be used alone or as acombination of two or more kinds thereof. The content of the carboxylicacid or a salt thereof in the plating solution of the present inventionis not particularly limited, and for example, is from 5 to 90 g/L, andpreferably from 10 to 60 g/L. In the present invention, the hydroxygroup in the carboxy group is not counted as the hydroxy group.

The conductive salt used in the plating solution of the presentinvention is potassium sulfate and ammonium sulfate. The content ofpotassium sulfate and ammonium sulfate in the plating solution of thepresent invention is not particularly limited, and for example, is from100 to 300 g/L, and preferably from 120 to 240 g/L, in terms of totalamount. The mass ratio of potassium sulfate and ammonium sulfate((potassium sulfate)/(ammonium sulfate)) may be from 0.5 to 60, andpreferably from 1.0 to 30. In the case where the mass ratio (potassiumsulfate)/(ammonium sulfate) is in the range, good covering power can beobtained, and a chromium plating film can be formed on an article havinga complicated shape to be plated over a low current density portion.

The pH buffer used in the plating solution of the present invention isnot particularly limited, examples of which include boric acid, sodiumborate, potassium borate, phosphoric acid, and dipotassium hydrogenphosphate, and boric acid and sodium borate are preferred. The pH buffermay be used alone or as a combination of two or more kinds thereof. Thecontent of the pH buffer in the plating solution of the presentinvention is not particularly limited, and for example, is from 30 to150 g/L, and preferably from 50 to 110 g/L.

The sulfur-containing organic compound used in the plating solution ofthe present invention is a combination of saccharin or a salt thereofand a sulfur-containing organic compound having an allyl group. Examplesof the saccharin or a salt thereof include saccharin and sodiumsaccharinate. Examples of the sulfur-containing organic compound havingan allyl group include sodium allylsulfonate, allylthiourea, ammonium2-methylallylsulfonate, and allyl isothiocyanate. The sulfur-containingorganic compound having an allyl group maybe used alone or as acombination of two or more kinds thereof, and sodium allylsulfonateand/or allylthiourea are preferred. Preferred examples of thecombination as the sulfur-containing organic compound include sodiumsaccharinate and sodium allylsulfonate. The content of thesulfur-containing organic compound in the plating solution of thepresent invention is not particularly limited, and for example, is from0.5 to 10 g/L, and preferably from 2 to 8 g/L.

The plating solution of the present invention may further containascorbic acid, sodium ascorbate, hydrogen peroxide, polyethylene glycol,and the like.

The pH of the plating solution of the present invention is notparticularly limited, as far as the solution is acidic, and for example,is preferably from 2 to 4.5, and more preferably from 2.5 to 4.0.

The preparation method of the plating solution of the present inventionis not particularly limited, and for example, the solution can beprepared in such a manner that the trivalent chromium compound, thecomplexing agent, the conductive salt, and the pH buffer are added towater at from 40 to 50° C. and dissolved therein by mixing, then thesulfur-containing organic compound is added thereto and mixed therein,and finally the pH thereof is controlled with sulfuric acid, aqueousammonia, or the like.

Chromium plating can be formed on an article to be plated byelectroplating the article to be plated with the plating solution of thepresent invention in the same manner as for the ordinary chromiumplating solution.

The condition of the electroplating is not particularly limited, and forexample, the electroplating may be performed at a bath temperature offrom 30 to 60° C., with carbon or iridium oxide as an anode, at acathode current density of from 2 to 20 A/dm², for a period of from 1 to15 minutes.

Examples of the article to be plated capable of being electroplatedinclude a metal, such as iron, stainless steel, and brass, and a resin,such as ABS and PC/ABS. The article to be plated may be subjected to atreatment, such as copper plating and nickel plating, in advance beforethe treatment with the plating solution of the present invention.

The chromium-plated product thus obtained becomes a chromium-platedproduct having an appearance, throwing power, and a deposition rate thatare equivalent to the use of hexavalent chromium.

As another embodiment of the plating solution of the present invention,a trivalent chromium plating solution that has a high plating depositionrate, has good color tone and good corrosion resistance, and ispractical is then described.

As the complexing agent of the aforementioned plating solution of thepresent invention, a carboxylic acid having two or more carboxy groupshaving a number of carbon atoms of 4 or more or a salt thereof isfurther used in combination, in addition to the aforementioned compound.Examples of the carboxylic acid having two or more carboxy groups havinga number of carbon atoms of 4 or more or a salt thereof include acarboxylic acid, such as adipic acid, phthalic acid, pimelic acid, andsebacic acid, and a salt of the carboxylic acid. In the case where acombination of the carboxylic acid having two or more hydroxy groups andtwo or more carboxy groups or a salt thereof and the carboxylic acidhaving two or more carboxy groups having a number of carbon atoms of 4or more or a salt thereof is used as the complexing agent, the two kindsof the complexing agents each may be used alone or as a combination oftwo or more kinds thereof. In the case where a combination of thecarboxylic acid having two or more hydroxy groups and two or morecarboxy groups or a salt thereof and the carboxylic acid having two ormore carboxy groups having a number of carbon atoms of 4 or more or asalt thereof is used as the complexing agent, the total content of thecomplexing agents in the plating solution of the present invention isnot particularly limited, and for example, is from 5 to 90 g/L, andpreferably from 10 to 60 g/L, in terms of total of all the complexingagents.

As the sulfur-containing organic compound of the aforementioned platingsolution of the present invention, a sulfonic acid having a vinyl groupor a salt thereof may further be used in combination, in addition to theaforementioned compounds. Examples of the sulfonic acid having a vinylgroup or a salt thereof include sodium vinylsulfonate, methylvinylsulfonate, and polyvinylsulfonic acid. In the case where acombination of the saccharin or a salt thereof, the sulfonic acid havingan allyl group or a salt thereof, and the sulfonic acid having a vinylgroup or a salt thereof is used as the sulfur-containing organiccompound, the three kinds of the sulfur-containing organic compoundseach may be used alone or as a combination of two or more kinds thereof.The content of the sulfur-containing organic compounds in the platingsolution of the present invention is not particularly limited, and forexample, is from 0.5 to 10 g/L, and preferably from 2 to 8 g/L, in termsof total of all the sulfur-containing organic compounds.

The plating solution of the present invention that uses the complexingagents and the sulfur-containing organic compounds described above canbe prepared in the preparation method described above. Chromium platingcan be formed therewith on an article to be plated in the methoddescribed above.

The chromium-plated product thus obtained becomes chromium platinghaving a color tone that is equivalent to the use of hexavalentchromium, and having high corrosion resistance and high practicality.Accordingly, the chromium-plated product is favorably applied tocomponents of automobiles, motorcycles, faucets, and the like, which aredemanded to have corrosion resistance.

An ordinary trivalent chromium plating solution contains iron or cobaltfor the enhancement of the throwing power for a low current density, butthe plating solution of the present invention can have enhanced throwingpower without the addition of iron and/or cobalt. A plating solutioncontaining iron or cobalt has a tendency that the corrosion resistanceof the plating film is decreased due to codeposition of iron or cobalttherein. Accordingly, it is preferred that the plating solution of thepresent invention contains substantially no iron and/or cobalt. Theplating solution of the present invention that contains substantially noiron and/or cobalt means that the content of iron and/or cobalt is 2 ppmor less, preferably 1 ppm or less, and more preferably 0.5 ppm or less.The amount of iron and/or cobalt can be analyzed by the ICP-MS method,the atomic absorption spectrometry, or the like.

In the case where the plating solution of the present invention containssubstantially no iron and/or cobalt, the resulting chromium-platedproduct also contains substantially no iron and/or cobalt. Thechromium-plated product of the present invention that containssubstantially no iron and/or cobalt means that the content of ironand/or chromium in the chromium plating is less than 0.5% by atom, andpreferably 0.4% by atom or less. The amount of iron and/or cobalt can beanalyzed by EDS, XPS, or the like.

EXAMPLES

The present invention will be described in detail with reference toexamples and comparative examples below, but the present invention isnot limited to the examples.

Example 1 Chromium Plating

The components shown in Table 1 were dissolved in water to prepare atrivalent chromium plating solution. The trivalent chromium platingsolution was subjected to the Hull cell test using a brass plate havingnickel plating formed thereon. The condition of the Hull cell test was acurrent of 4 A and a plating time of 3 minutes. After plating, thedistance of the deposition of the plating film from the left end of thebrass plate was measured as shown in FIG. 1, and was designated as thethrowing power. The film thickness at the position of the brass platecorresponding to a current density of 8 ASD was measured by the X-rayfluorescent spectrometry. The appearance after plating was evaluated interms of the L value, the a value, and the b value with acolor-difference meter (produced by Konica Minolta, Inc.). The resultsare shown in Table 1.

TABLE 1 Composition Composition Composition Composition CompositionComposition Composition of plating solution 1 2 3 4 5 6 Basic chromiumsulfate (g/L) 64 64 64 64 64 64 Diammonium tartarate (g/L) *¹ 30 30 3030 30 — Ammonium lactate (g/L) *² — — — — — 30 Potassium sulfate (g/L)150 150 150 150 150 150 Ammonium sulfate (g/L) 20 20 20 20 20 20 Boricacid (g/L) 80 80 80 80 80 80 Sodium Saccharate (g/L) 4 4 4 4 4 4 Sodiumallylsulfonate (36%) (ml/L) 1.58 — — — — 1.58 Sodium vinylsulfonate(27.5%) (m/L) — 1 — — — — Sodium propinesulfonate (20%) (ml/L) — — 1 — —— Allylthiourea (ppm) — — — 10 20 — pH of plating solution 3.4 3.4 3.43.4 3.4 3.4 Bath temperature (° C.) 45 45 45 45 45 45 Throwing power(mm) 75 75 —* 75 74 74 Film thickness (μm) 0.15 0.07 —* 0.13 0.15 0.08Appearance L value 82.38 75.47 —* 82.60 82.40 80.01 Appearance a value−0.76 −0.29 —* −0.76 −0.72 −0.75 Appearance b value −0.42 1.94 —* −0.22−0.33 0.23 —*: no plating deposited *¹ two hydroxy groups and twocarboxy groups *² one hydroxy group and one carboxy group note:Hexavalent chromium plating has L value of 84, a value of −1.0, and bvalue of −1.0.

It was found from the results that in the case where the trivalentchromium plating solution used diammonium tartarate (having two hydroxygroups and two carboxy group) as the complexing agent, and a combinationof sodium saccharate and sodium allylsulfonate or allylthiourea (i.e.,the sulfur-containing organic compound having an allyl group) as thesulfur-containing organic compound (Compositions 1, 4, and 5), theappearance was equivalent to hexavalent chromium plating, and the filmthickness (i.e., the deposition rate) was approximately twice the casewhere these compounds were not used.

The contents of iron and cobalt in the trivalent chromium platingsolutions each were less than 0.5 ppm, as measured by the ICP-MS method.The contents of iron and cobalt in the resulting chromium plating eachwere less than 0.4% by atom, as measured by the EDS elemental analysis.

Example 2 Chromium Plating

The components shown in Table 2 were dissolved in water to prepare atrivalent chromium plating solution. The trivalent chromium platingsolution was subjected to the Hull cell test using a brass plate havingnickel plating formed thereon. The condition of the Hull cell test was acurrent of 4 A and a plating time of 3 minutes. After plating, thedistance of the deposition of the plating film from the left end of thebrass plate was measured. The results are shown in Table 2. Therelationship between the ratio (potassium sulfate)/(ammonium sulfate)and the throwing power is shown in FIG. 2

TABLE 2 Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo- Compo-sition sition sition sition sition sition sition sition sitionComposition of plating solution 7 8 9 10 11 12 13 14 15 Basic chromiumsulfate (g/L) 64 64 64 64 64 64 64 64 64 Diammonium tartarate (g/L) 3030 30 30 30 30 30 30 30 Potassium sulfate (g/L) 150 150 150 120 85 60 4020 5 Ammonium sulfate (g/L) 20 5 40 60 85 120 150 150 150 Boric acid(g/L) 80 80 80 80 80 80 80 80 80 Sodium Saccharate (g/L) 4 4 4 4 4 4 4 44 Sodium allylsulfonate (36%) (ml/L) 1.58 1.58 1.58 1.58 1.58 1.58 1.581.58 1.58 pH of plating solution 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4Bath temperature (° C.) 45 45 45 45 45 45 45 45 45 Throwing power (mm)75 79 71 70 69 67 64 62 62

It was found that in the case where plating was performed with theaforementioned trivalent chromium plating solutions, the appearance andthe film thickness were substantially the same in all the compositions,but the throwing power became better with a larger ratio (potassiumsulfate)/(ammonium sulfate), and the throwing power became particularlygood with the ratio of from 1.0 to 30.

The contents of iron and cobalt in the trivalent chromium platingsolutions each were less than 0.5 ppm, as measured by the ICP-MS method.The contents of iron and cobalt in the resulting chromium plating eachwere less than 0.4% by atom, as measured by the EDS elemental analysis.

Example 3 Chromium Plating

The components shown in Table 3 were dissolved in water to prepare atrivalent chromium plating solution. The trivalent chromium platingsolution was subjected to the Hull cell test using a brass plate havingnickel plating formed thereon. The condition of the Hull cell test was acurrent of 4 A and a plating time of 3 minutes. After plating, thedistance of the deposition of the plating film from the left end of thebrass plate was measured as shown in FIG. 1, and was designated as thethrowing power. The film thickness at the position of the brass platecorresponding to a current density of 8 ASD was measured by the X-rayfluorescent spectrometry. The appearance after plating was evaluated interms of the L value, the a value, and the b value with acolor-difference meter (produced by Konica Minolta, Inc.). The resultsare shown in Table 3.

TABLE 3 Composition Composition Composition Composition CompositionComposition Composition of plating solution 16 17 18 19 20 21 Basicchromium sulfate (g/L) 64 64 64 64 64 64 Diammonium tartarate (g/L) 3030 30 30 30 30 Adipic acid (g/L) — — 1 5 — — Phthalic acid (g/L) 1 5 — —— — Glycine (g/L) — — — — — 1 Potassium sulfate (g/L) 150 150 150 150150 150 Ammonium sulfate (g/L) 20 20 20 20 20 20 Boric acid (g/L) 80 8080 80 80 80 Sodium Saccharate (g/L) 4 4 4 4 4 4 Sodium allylsulfonate(36%) (ml/L) 3.6 3.6 7.2 7.2 3.6 3.6 Sodium vinylsulfonate (25%) (ml/L)3 3 5 5 3 3 pH of plating solution 3.4 3.4 3.4 3.4 3.4 3.4 Bathtemperature (° C.) 45 45 45 45 45 45 Throwing power (mm) 76 75 75 74 7575 Film thickness (μm) 0.14 0.11 0.14 0.12 0.15 0.13 Appearance L value82.0 81.8 82.2 82.0 82.8 81.7 Appearance a value −0.78 −0.72 −0.77 −0.74−0.64 −0.74 Appearance b value −0.25 −0.10 −0.33 −0.17 −0.58 −0.14 note:Hexavalent chromium plating has L value of 84, a value of −1.0, and bvalue of −1.0.

It was found from the results that in the case where the trivalentchromium plating solution used a combination of a carboxylic acid havingtwo or more hydroxy groups and two or more carboxy groups or a saltthereof and a carboxylic acid having two or more carboxy groups having anumber of carbon atoms of 4 or more or a salt thereof as the complexingagent, and a combination of saccharate or a salt thereof, a sulfonicacid having an allyl group or a salt thereof, and a sulfonic acid havinga vinyl group or a salt thereof as the sulfur-containing organiccompound, the appearance was equivalent to hexavalent chromium plating.

The contents of iron and cobalt in the trivalent chromium platingsolutions each were less than 0.5 ppm, as measured by the ICP-MS method.The contents of iron and cobalt in the resulting chromium plating eachwere less than 0.4% by atom, as measured by the EDS elemental analysis.

Example 4 CASS Test

Trivalent chromium plating solutions having the compositions 16, 18, 20,and 21 shown in Table 3 were prepared. With each of the trivalentchromium plating solutions, chromium plating was formed on a copperplate having nickel plating (2 μm) thereon under condition of a bathtemperature of 45° C. and a current density of 8 A/dm² for 3 minutes, soas to prepare a test piece. The test piece was subjected to the CASStest (according to JIS H8502). The micrographs of the test pieces afterthe CASS test for 24 hours are shown in FIG. 3.

It was found from the results of the CASS test that in the case wherethe trivalent chromium plating solution used a combination of acarboxylic acid having two or more hydroxy groups and two or morecarboxy groups or a salt thereof and a carboxylic acid having two ormore carboxy groups having a number of carbon atoms of 4 or more or asalt thereof as the complexing agent, and a combination of saccharate ora salt thereof, a sulfonic acid having an allyl group or a salt thereof,and a sulfonic acid having a vinyl group or a salt thereof as thesulfur-containing organic compound, the corrosion pores were finelydispersed to enhance the corrosion resistance.

INDUSTRIAL APPLICABILITY

The trivalent chromium plating solution of the present invention can beapplied to various purposes as similar to plating with hexavalentchromium.

1. A trivalent chromium plating solution, comprising: a trivalentchromium compound, a first complexing agent, potassium sulfate andammonium sulfate as a conductive salt, a pH buffer, and a firstsulfur-containing organic compound, wherein the first complexing agentis a carboxylic acid having two or more hydroxy groups and two or morecarboxy groups or a salt thereof, and the first sulfur-containingorganic compound is a combination of saccharin or a salt thereof and asulfur-containing organic compound having an allyl group.
 2. Thetrivalent chromium plating solution according to claim 1, wherein thecarboxylic acid having two or more hydroxy groups and two or morecarboxy groups or a salt thereof is diammonium tartarate.
 3. Thetrivalent chromium plating solution according to claim 1, wherein thetrivalent chromium plating solution has a mass ratio of potassiumsulfate and ammonium sulfate ((potassium sulfate)/(ammonium sulfate)) offrom 1.0 to
 30. 4. The trivalent chromium plating solution according toclaim 1, wherein the sulfur-containing organic compound having an allylgroup is sodium allylsulfonate and/or allylthiourea.
 5. The trivalentchromium plating solution according to claim 1, further comprising: asecond complexing agent, and a second sulfur-containing organiccompound, wherein the second complexing agent is a carboxylic acidhaving two or more carboxy groups having a number of carbon atoms of 4or more or a salt thereof, and the second sulfur-containing organiccompound is a sulfonic acid having a vinyl group or a salt thereof. 6.The trivalent chromium plating solution according to claim 5, whereinthe carboxylic acid having two or more carboxy groups having a number ofcarbon atoms of 4 or more or a salt thereof is phthalic acid or adipicacid.
 7. The trivalent chromium plating solution according to claim 5,wherein the sulfonic acid having a vinyl group or a salt thereof issodium vinylsulfonate.
 8. The trivalent chromium plating solutionaccording to claim 1, wherein the trivalent chromium plating solutioncomprises substantially no iron and/or cobalt.
 9. A method forchromium-plating on an article, the method comprising: electroplating anarticle with the trivalent chromium plating solution according toclaim
 1. 10. A chromium-plated product, comprising: an article,electroplated with the trivalent chromium plating solution according toclaim
 1. 11. The chromium-plated product according to claim 10, whereinthe chromium plating comprises substantially no iron and/or cobalt.